Why The Hell... (random rant)

Peter Dohm writes:

Some of you guys are a *lot* more trusting than I am. I was really hoping
for some of the current airmen to say this, but most have only nibbled
around the edges--so here goes:

The magnetic compass has exactly one thing in its favor, and that is just
plain old Brute Reliability.

It requires no power from the aircraft's systems, it is not subject to
happenstance or whim concerning any transmitting stations, and wide spread
interference with (the) signal is unimaginable.

It's already so inaccurate without interference that that's bad enough. There
are plenty of spots on charts where the compass will be 6-8 degrees off even
from the already irrgular declination over larger areas.

Anyway, if you push this concept to its limit, you should be able to complete
a trip without an engine, since engines are not 100% reliable. Obviously,
that's not a practical reality, and at some point you have to recognize that a
compass alone, no matter how reliable in the sense of always working to some
extent, may simply not be enough to get you home.

--
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Peter Dohm wrote:

The magnetic compass has exactly one thing in its favor, and that is just
plain old Brute Reliability.

I like the mag compass too. While they don't rely on any aircraft systems
to work, there are things that can cause them to be innacurate. The earths
magnetic field varies in strength and isn't all that constant in all
locations (and it's getting worse).

I was reading Lindbergh's book about his transatlantic flight and at one
point, both his whiskey compass and Earth Inductor Compass were just wobbling
around uselessly. Eventually, they both started working again on their own,
but he was guessing at his heading for nearly an hour.

I can relate. I have a video that I took on a cross country flight, of my
mag compass doing rapid 360s. It lasted about 5 min. and there were no
magnetic anomolies listed on the chart in that area. There were also no UFO
sightings reported that day :-))

John Galban=====N4BQ (PA28-180)

--
Message posted via AviationKB.com
http://www.aviationkb.com/Uwe/Forums...ation/200704/1

writes:

Would be simple to have DGs slaved to true north automatically, given
a GPS or Inertial platform.

This is switch-selectable in some aircraft, I think because true north is much
easier to deal with when flying over long routes without navaid coverage
during which latitudes and longitudes must be dealt with directly (e.g.,
transatlantic flights).

--
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"Kev" wrote in message
ups.com...

VORs are allowed to get +/- 6 degrees off. Seems a lot, but it's
only a handful of miles off-course over most VORs' ranges. Anyway,
see:

http://www.naco.faa.gov/index.asp?xml=naco/faq#q2h


"The magnetic variation of the earth changes at a rate of 50.27 seconds of
arc per year." That seems a bit low to me. At that rate six degrees of
change would take 430 years.

Steven P. McNicoll writes:

"The magnetic variation of the earth changes at a rate of 50.27 seconds of
arc per year." That seems a bit low to me. At that rate six degrees of
change would take 430 years.

It changes at a variable rate, and once it approaches a degree, charts and
navaids have to change. It's a tremendous amount of extra work, an additional
source of error, and an imprecise and fickle basis for navigation.

--
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"Mxsmanic" wrote

It changes at a variable rate, and once it approaches a degree, charts and
navaids have to change. It's a tremendous amount of extra work, an
additional
source of error, and an imprecise and fickle basis for navigation.


I came to aviation from boats. In that field of navigation, in my part of
the world we mostly use true north as reference, but I've seen charts using
magnetic north, e.g. in the Caribbean where the magnetic variation is
considerable (around 15 degrees). Also, all boat GPSes can be set to show
either true or magnetic north, so using magnetic north is obviously a viable
method also at sea. So, let me give some arguments why using magnetic north
on aviation charts is not as stupid as it may sound.

1. A GPS does not show a reliable heading unless the aircraft is moving
(unless the GPS is of a very advanced type with dual antennas, not widely
used in aircraft). Before every takeoff the DG needs to be set, and the
magnetic compass enables this also when the aircraft is stationary.

2. The extra work referred to by Mx above is the same as would otherwise
have to be done in-flight in each aircraft, adding to the crew's workload
and introducing many possibilities for errors. (Mariners actually do this
en-route, but they have much more time to do the calculations than pilots
have.) For aviation, it's safer to do the calculations and corresponding
changes to charts and navaid data at a central source where they can be
quality-controlled much more extensively than what the co-pilot can do in an
aircraft in-flight. Charts need to be updated frequently anyway, regardless
of changes in magnetic variation, so it's not a big deal.

3. Even though electronic or inertial systems would allow navigation without
any reference to magnetic north, making the above mentioned calculations
unnecessary, experience and tradition so far points to the conclusion that a
magnetic compass and the corresponding magnetic designations on charts are
still useful. Note that when the magnetic compass is most critically needed,
i.e. in case of a failure of the electronic navigation systems, that's also
the time when the pilots have least time and opportunity to perform the
extra calculations that would be required if the chart data were given in
true north.

4. The magnetic compass system gives a simple foolproof method to ensure the
aircraft is lined up on the correct runway, anywhere in the world. Last
August, 49 real people died at KLEX in an accident that might have been
prevented by this simple check.

5. Omitting the magnetic compass would make the aircraft totally dependent
on external sources for all navigation other than chart-based VFR (unless it
has an inertial navigation system, which is obviously a much more expensive
solution viable only for large aircaft). This is so far not considered
acceptable.

On Apr 4, 12:22 pm, "Steven P. McNicoll"
wrote:
"Kev" wrote in message

ups.com...
VORs are allowed to get +/- 6 degrees off. [..]
http://www.naco.faa.gov/index.asp?xml=naco/faq#q2h

"The magnetic variation of the earth changes at a rate of 50.27secondsof
arc peryear." That seems a bit low to me. At that rate six degrees of
change would take 430 years.

Yeah, weird, eh? So I ran across this section of a site explaining
that it's often really a lot faster than that (2-25 years per degree).

http://www.geocities.com/magnetic_declination/#FACTORS

The "Local magnetic anomalies" section mentions the Ramapo area by me.

The "Where were/are/will be the magnetic poles? " talks about the
movement.

And the section about the "reversing Earth" theory is just plain
terrifying :-) (Not just reversal of the poles... but the entire
crust rotating upside down in a matter of days... ouch!!)

Kev

And the section about the "reversing Earth" theory is just plain
terrifying :-) (Not just reversal of the poles... but the entire
crust rotating upside down in a matter of days... ouch!!)

"It must be true - I found it on the internet"

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

On Apr 4, 9:06 am, "Kev" wrote:
On Apr 4, 2:32 am, Tauno Voipio wrote:
It's a nuisance for maintenance, but it frees
the pilot from calculating the variation at
the operative time. Just to off-load the pilot.

A good answer. Same for winds in flight, etc. Everything is based on
the magnetic to make it easier for the pilot. [..]

Oops! I can't believe I wrote that about winds in flight. Of course
they're true, so they can be used over a wider area. Winds at the
_airport_ are magnetic, is what I meant to say... so the pilot doesn't
have to convert while landing.

Hmm. So if we switched to true North navigation, then runways would
all need repainting... but at least they and the VORs wouldn't have to
be changed ever again ;-) Seems like something the government would
come up with to save money!

Kev

It's a nuisance for maintenance, but it frees
the pilot from calculating the variation at
the operative time. Just to off-load the pilot.

A good answer. Same for winds in flight, etc. Everything is based on
the magnetic to make it easier for the pilot. [..]

Oops! I can't believe I wrote that about winds in flight. Of course
they're true, so they can be used over a wider area. Winds at the
_airport_ are magnetic, is what I meant to say... so the pilot doesn't
have to convert while landing.

Hmm. So if we switched to true North navigation, then runways would
all need repainting... but at least they and the VORs wouldn't have to
be changed ever again ;-) Seems like something the government would
come up with to save money!

Kev

I was wondering whether to mention something about the probable long term
causes of drift in variation--and then I finally noticed the smiley.

Thanks :-)))
Peter